I was there last summer. I went to 3 of the parties. I had been to mykonos many times but not for the xlsior festival.
Keep in mind that each party will cost you 55 euros entry and NO drink with that. Only redbull is served and they charge 10 euros for one can and 5 euros for a half litre bottle of water!!!
It's probably better to buy the bracelet from the start which gives you access to all the parties. I have no idea how much it costs though and they are usually snatched up quickly.
The first party i went to was a spanish themed party at cavo paradiso..
All of a sudden there were like 4000 gay spanish guys there..
The parties were pretty amazing. But seriously these parties are just a big excuse to do drugs and have sex in the wc...
The last party of the festival was amazing there were like 5000 people maybe more. I talked to francois sagat and then there were fireworks over the sea..
im not a drug user though so i doubt i'll be going to the parties this year..
The island is great with or without the xlsior festival...